A mechanical face seal, which may be employed for example in a dishwasher, may include a stationary seal head assembly that mates with and seals against a rotating seal seat assembly. Typically, the seal seat assembly mounts to an impeller that is driven, via a shaft, by a pump. It is common for the impeller to be made of a plastic material. Such a conventional face seal assembly performs very well at sealing out water, and prevents the impeller from overheating because the water adjacent to the seal absorbs frictional heat generated by the seal. A concern arises, however, with these conventional face seals, when the pump is run dry—that is, the pump is run, but a water supply valve is closed, thus preventing water from flowing adjacent to the face seal for cooling the seal. This situation can happen, for example, in a dishwasher, when a dishwasher is installed, but the water supply valve is not opened. If the dishwasher is then run, there is no water adjacent the seal to carry away the frictional heat generated by the seal operation. The frictional heat then builds up, and some of the heat will transfer into the impeller, which also will have no water surrounding it to carry away heat. After operating in this way for a relatively short time, the heat in the impeller can build up to the point where it will partially melt and possibly decouple from an impeller hub. This can destroy the impeller pumping ability and possibly create a water leak path.
Thus, it is desirable to have a face seal for a rotating shaft that will adequately seal water, and can also significantly reduce the chance of heat build up damaging a mating component, such as an impeller, even if the face seal is run in a dry environment.